Eur. J. Mineral., 32, 77–87, 2020 https://doi.org/10.5194/ejm-32-77-2020 © Author(s) 2020. This work is distributed under the Creative Commons Attribution 4.0 License.
Adding complexity to the garnet supergroup: monteneveite, 5+ 3+ 2+ Ca3Sb2 (Fe2 Fe )O12, a new mineral from the Monteneve mine, Bolzano Province, Italy
Andreas Karlsson1, Dan Holtstam1, Luca Bindi2, Paola Bonazzi2, and Matthias Konrad-Schmolke3 1Department of Geosciences, Swedish Museum of Natural History, P.O. Box 50007, 10405 Stockholm, Sweden 2Dipartimento di Scienze della Terra, Università degli Studi di Firenze, Via La Pira 4, 50121 Florence, Italy 3Department of Earth Sciences, University of Gothenburg, P.O. Box 460, 40530 Gothenburg, Sweden Correspondence: Andreas Karlsson ([email protected])
Received: 15 November 2019 – Published: 21 January 2020
5+ 3+ 2+ Abstract. Monteneveite, ideally Ca3Sb2 (Fe2 Fe )O12, is a new member of the garnet supergroup (IMA 2018-060). The mineral was discovered in a small specimen belonging to the Swedish Museum of Natural History coming from the now abandoned Monteneve Pb–Zn mine in Passiria Valley, Bolzano Province, Alto Adige (South Tyrol), Italy. The specimen consists of mainly magnetite, sphalerite, tetrahedrite-(Fe) and oxy- calcioroméite. Monteneveite occurs as black, subhedral crystals with adamantine lustre. They are equidimen- sional and up to 400 µm in size, with a subconchoidal fracture. Monteneveite is opaque, grey in reflected light, and isotropic under crossed polars. Measured reflectance values (%) at the four COM wavelengths are 12.6 −2 (470 nm), 12.0 (546 nm), 11.6 (589 nm) and 11.4 (650 nm). The Vickers hardness (VHN100) is 1141 kg mm , corresponding to H = 6.5–7, and the calculated density is 4.72(1) g cm−3. A mean of 10 electron microprobe analyses gave (wt %) CaO 23.67, FeO 3.75, Fe2O3 29.54, Sb2O5 39.81, SnO2 2.22, ZnO 2.29, MgO 0.15, MnO 0.03 and CoO 0.03. The crystal chemical formula calculated on the basis of a total of eight cations and 12 anions, and taking into account the available structural and spectroscopic data, is (Ca2.97Mg0.03)6=3.00 5+ 4+ 3+ 3+ 2+ (Sb1.73Sn0.10Fe0.17)6=2.00(Fe2.43Fe0.37Zn0.20)6=3.00O12. The most significant chemical variations encountered in the sample are related to a substitution of the type Y Sn4++ZFe3+→Y Sb5++ZFe2+. Mössbauer data obtained at RT and 77 K indicate the presence of tetrahedrally coordinated Fe2+. Raman spectroscopy demonstrates that there is no measurable hydrogarnet component in monteneveite. The six strongest Bragg peaks in the powder X-ray diffraction pattern are [d (Å), I (%), (hkl)]: 4.45, 100, (220); 3.147, 60, (400); 2.814, 40, (420); 2.571, 80, (422); 1.993, 40, (620); 1.683, 60, (642). Monteneveite is cubic, space group Ia-3d, with a = 12.6093(2) Å, V = 2004.8(1) Å3, and Z = 8. The crystal structure was refined up to R1 = 0.0197 for 305 reflections with Fo > 4σ (Fo) and 19 parameters. Monteneveite is related to the other Ca-, Sb- and Fe-bearing, nominally Si-free members of the bitikleite group, but it differs in that it is the only known garnet species with mixed trivalent and divalent cations (2 : 1) at the tetrahedral Z site. Textural and mineralogical evidence suggests that monteneveite formed during peak metamorphism (at ca. 600 ◦C) during partial breakdown of tetrahedrite-(Fe) by reactions with carbonate, under relatively oxidizing conditions. The mineral is named after the type locality, the Monteneve (Schneeberg) mine.
Published by Copernicus Publications on behalf of the European mineralogical societies DMG, SEM, SIMP & SFMC. 78 A. Karlsson et al.: Monteneveite, a new Sb-bearing member of the garnet supergroup
1 Introduction men originates; the original label simply states “Schneeber- git, Schneeberg, Südtirol”. The sample could potentially be The garnet supergroup, with the general formula {X3} from Bockleitner Halde (“Bockleitner dump”), from where [Y2](Z3)ϕ12, includes all minerals isostructurally with gar- Brezina (1880) described a very similar mineral assemblage net regardless of what elements occupy the four atomic sites; in his work on Schneebergit; however, he reported his spec- i.e. the supergroup includes several chemical classes (Grew imen as being associated with anhydrite and gypsum, which et al., 2013). In particular, in recent years, several new mem- are not found in the present material. This was later corrected bers of the garnet supergroup that are nominally Si-free and by von Elterlein (1891), who pointed out that the supposed contain elements such as Sb and Sn have been approved by sulfates are in fact calcite. Brezina (1880) makes no mention the Commission on New Minerals, Nomenclature and Clas- of tetrahedrite in association with Schneebergit, even though sification (CNMNC) of the International Mineralogical As- the general description of oxycalcioroméite as 0.5 to 1 mm sociation (IMA). honey-yellow octahedra fits well with the monteneveite holo- During an examination of roméite group specimens from type specimen. various localities, a sample from the Monteneve (Schnee- Monteneveite occurs mainly with magnetite, oxycal- berg) mine, Passiria Valley, Bolzano Province, Alto Adige cioroméite, tetrahedrite-(Fe), sphalerite and chalcopy- (South Tyrol), Italy (46◦5304600 N, 11◦1004600 E, ∼ 2300 m rite (Figs. 2 and 3). Sphalerite frequently contains above sea level), was found to contain a new mineral minute inclusions of chalcopyrite. The average compo- belonging to the garnet supergroup. The small sample sition of tetrahedrite-(Fe) (by means of five energy- (ca. 30 mm × 15 mm× 6 mm), labelled “Schneebergit” (iron- dispersive X-ray spectroscopy, EDS, point analyses) is bearing oxycalcioroméite) (Fig. 1), had been purchased (for Cu9.5Ag0.1(Fe1.4Zn0.6)Sb4.1S12.9. Oxycalcioroméite occurs 15 Imperial German Goldmarks) by Hjalmar Sjögren from as 0.1–1 mm honey-yellow to orange-brown euhedral crys- Dr. F. Krantz in 1897. Sjögren’s collection was then donated tals, and the composition is Ca1.4Fe0.4Sb1.8Sn0.2O7 (mean of to the Swedish Museum of Natural History in 1901. eight EDS point analyses). Magnetite occurs both as euhedral This new member of the garnet supergroup, with the ideal crystals and as anhedral grains enveloped by monteneveite. 5+ 3+ 2+ formula Ca3Sb2 (Fe2 Fe )O12, was named monteneveite In addition, the sample contains minor amounts of antho- after the type locality, and both the mineral and mineral phyllite, gudmundite and genplesite; the latter represents the name have been approved by the IMA CNMNC (2018–060). second recorded occurrence after that from the Talnakh Cu– The name complies with the IMA nomenclature for the gar- Ni deposit, Norilsk, Russian Federation (Pekov et al., 2018). net supergroup (Grew et al., 2013), i.e. a new root name is The sample is quite vuggy, but we suspect that it has been used rather than suffixes. The holotype specimen is deposited treated with dilute acid at some point in order to dissolve the at the Swedish Museum of Natural History, Department of carbonates (mainly calcite) to reveal the oxycalcioroméite Geosciences, Box 50007, SE-10405 Stockholm, Sweden, un- crystals. der collection number GEO-NRM catlogue no. HS3903.
3 Appearance and physical properties 2 Occurrence and paragenesis Monteneveite forms subhedral crystals, with a maximum The Monteneve area has been mined for its lead and zinc size of up to about 0.4 mm and essentially equidimensional ores as far back as the 13th century all the way until the last in shape. The grains are commonly fractured, with radial mine closed in 1985, and it consists of several hundred small patterns. The macroscopic colour is black (opaque), with mines and adits (Tasser, 1994). brownish black streak and adamantine lustre. The crys- The ore deposits occur as bands of sulfide minerals (pri- tals are non-fluorescent and non-magnetic. Microintenda- marily sphalerite, pyrrhotite, galena and chalcopyrite) and tion hardness was measured with a Shimadzu M-type hard- are part of the Ötztal–Stubai polymetamorphic complex ness tester, yielding VHN100 = 1072–1246, with an average (ÖSC). The ores are found conformable within two distinct of 1141 kg mm−2 (11.2 GPa) from four indentations, corre- stratigraphic horizons of the metasedimentary Breonie met- sponding to a Mohs hardness of 6.5–7. There is no observed alliferous series (Frizzo et al., 1982). The Monteneve area cleavage or parting; the fracture is subconchoidal. The den- exhibits a complex metamorphic history and some authors sity could not be empirically determined due to paucity of argue for a Cretaceous age of the metamorphism (Miller et material. Using the empirical formula, the calculated density al., 1967), but others advocate an early Alpine metamorphic is 4.72(1) g cm−3 for single-crystal X-ray diffraction data. age (see discussion in Sassi et al., 1985). Modern estimates In reflected-light microscopy, the colour is grey (Fig. 2). of T and P on mineral assemblages in country rocks sug- The overall reflectivity is significantly lower than that of as- gest peak metamorphic conditions of 600 ◦C and 8–10 kbar, sociated magnetite, but similar to that of oxycalcioroméite. respectively (Konzett et al., 2003). It is unknown from which The latter mineral, in contrast to monteneveite, shows many mine level and consequently geological unit the type speci- internal reflections. Monteneveite is isotropic under crossed
Eur. J. Mineral., 32, 77–87, 2020 www.eur-j-mineral.net/32/77/2020/ A. Karlsson et al.: Monteneveite, a new Sb-bearing member of the garnet supergroup 79
Figure 1. Macroscopic photograph of monteneveite (mnv) in the holotype specimen, GEO-NRM catalogue no. HS3903, here seen Figure 3. Field-emission scanning electron microscope image together with almost euhedral, honey-yellow crystals of oxycal- (BSE) of monteneveite and associated phases. The image shows cioroméite and associated minerals. Field of view is 2.5 mm. © the same section as in Fig. 2 (with some rotation). Abbreviations: Lorin. All rights reserved. mnv – monteneveite; mag – magnetite; ocr – oxycalcioroméite; ttr – tetrahedrite-(Fe); sp – sphalerite; ccp – chalcopyrite. Sample GEO- NRM catalogue no. HS3903.
Table 1. Reflectance data for monteneveite. Interpolated values for COM (Commission of Ore Minerals) wavelengths are given in bold.
λ R % λ R % 400 14.3 560 11.8 420 13.6 580 11.7 440 13.2 589 11.6 460 12.8 600 11.6 470 12.6 620 11.5 480 12.5 640 11.4 500 12.3 650 11.4 520 12.1 660 11.4 540 12.0 680 11.3 546 12.0 700 11.3
Figure 2. Reflected-light microscope picture illustrating the textural 4 Chemical composition relations between the sulfides and oxides. Blue-filtered incandes- cent light. Abbreviations: mnv – monteneveite; mag – magnetite; The chemical composition of monteneveite was determined ocr – oxycalcioroméite; ttr – tetrahedrite-(Fe); sp – sphalerite; ccp using a JEOL JXA-8200 electron probe microanalyser – chalcopyrite. Sample GEO-NRM catalogue no. HS3903. (EPMA), fitted with five wavelength dispersive spectrome- ters, at the Institute of Earth and Environmental Sciences of Potsdam University. The instrument was operated at a polars. Reflectance data were measured in air with an working distance of 10 mm with an accelerating voltage of × AVASPEC-ULS2048 16 spectrometer attached to a Zeiss 15 kV and a beam current of 15 nA. The beam size was 2 µm. × Axiotron UV microscope (10 /0.20 Ultrafluar objective), Counting times were 20 s for major element peaks and 10 s using a halogen lamp (100 W) and a SiC (Zeiss no. 846) stan- left and right of the peak in order to determine background dard, with a measured field of 150 µm in diameter. The results counts during measurement. The elements were calibrated (average of 300 scans, integration time 30 ms) are given in on the following reference materials: Fe (almandine), Zn Table 1. (willemite), Sn (cassiterite), Sb (stibnite), Co (pentlandite), Mn (bustamite), Mg (almandine) and Ca (andradite). All standards were from Astimex except for andradite (Smithso- nian). Na, Si, Al, Ti, Zr and Cu were sought but not detected. www.eur-j-mineral.net/32/77/2020/ Eur. J. Mineral., 32, 77–87, 2020 80 A. Karlsson et al.: Monteneveite, a new Sb-bearing member of the garnet supergroup
Table 2. Chemical composition (wt %) of monteneveite from EPMA analyses.
Oxide wt %/ spot 1 2 3 4 5 6 7 8 9 10 Mean 2σ MgO 0.15 0.12 0.14 0.13 0.12 0.13 0.15 0.12 0.23 0.17 0.15 0.04 MnO 0.02 0.02 0.03 0.07 0.01 0.04 0.04 0.03 0.05 0.00 0.03 0.02 CaO 23.73 23.66 23.54 23.69 23.76 23.79 23.72 23.67 23.53 23.63 23.67 0.09 FeO 3.26 3.27 3.39 3.14 3.76 4.07 4.37 4.24 3.69 4.36 3.75 0.48 ∗ Fe2O3 30.00 29.96 29.44 30.45 30.98 30.01 28.80 29.39 27.82 28.58 29.54 0.94 Sb2O5 38.18 38.09 38.47 37.97 40.74 41.25 41.54 41.27 39.09 41.53 39.81 1.57 SnO2 4.00 3.92 3.57 4.13 0.41 0.35 0.67 0.69 3.96 0.47 2.22 1.80 ZnO 2.05 2.15 2.28 2.14 2.20 2.20 2.39 2.42 2.62 2.44 2.29 0.18 CoO 0.08 0.00 0.05 0.02 0.03 0.06 0.05 0.03 0.01 0.02 0.03 0.02 Total 101.47 101.19 100.91 101.74 102.01 101.90 101.73 101.86 101.00 101.20 101.49
∗ Fe3+/Fe2+ is calculated to obtain overall electroneutrality in the formula.
The analytical results obtained on the same crystal used ized confocal laser beam was of the order of 1 µm. Spec- for single-crystal X-ray diffraction (see below) are given in tra were generated in the range of 100 to 4000 cm−1 utiliz- Table 2 (average of 10 point analyses). The empirical for- ing a 600 grooves cm−1 grating and a thermoelectric cooled mula was calculated on the basis of eight cations, and the electron multiplier charge-coupled device (CCD) including a Fe3+/Fe2+ partition was calculated to obtain overall elec- front illuminated 1600 × 200 pixel chip. The spectral resolu- troneutrality. The chemical data reveal that in monteneveite tion on unoriented polished crystals of monteneveite embed- the Sb5+ (and Fe2+) contents exhibit negative covariance ded in epoxy resin was of the order of 1 cm−1. The wavenum- with respect to Sn4+ (and Fe3+), thus confirming that a ber calibration was performed using the 520.7 cm−1 Raman dzhuluite-type substitution occurs in the sample. band on a polished silicon wafer with a wavenumber accu- Taking the results from structural and spectroscopic in- racy usually better than 0.5 cm−1. Raman spectra of the sam- vestigation into account, the crystal chemical formula is the ple were collected through five acquisition cycles with single 5+ 4+ 3+ following: (Ca2.97Mg0.03)6=3.00(Sb1.73Sn0.10Fe0.17)6=2.00 counting times of 45 s in a near-back-scattered geometry. 3+ 2+ The Raman spectrum of monteneveite (Fig. 4) is similar (Fe2.43Fe0.37Zn0.20)6=3.00O12. The analytical totals, which are consistently higher than 100 %, are in accord with to that of the minerals in the bitikleite group (in particular the absence of H2O (see results from Raman spec- dzhuluite; Galuskina et al., 2013a). There is a strong band − troscopy) and other volatiles. The ideal formula is at 729 cm 1 related to symmetric stretching vibrations of 5+ 3+ 2+ 3+ {Ca3}[Sb ](Fe Fe )O12, which requires CaO 23.26, the tetrahedral Fe –O bond; the broadening of this band 2 2 2+ Sb2O544.73, Fe2O322.08, FeO 9.93 (total 100.00 wt %). could be attributed to the co-presence of Fe at the Z site The chemical composition of some minerals associated as also inferred from Mössbauer spectroscopy (see below). − with monteneveite was determined using an FEI Quanta 650 The medium-intensity band at 586 cm 1 could be caused by 3+ 5− field-emission scanning electron microscope, fitted with an asymmetric stretching vibrations in the (Fe O4) tetra- 80 mm2 X-MaxN Oxford Instruments EDS detector hosted at hedra but is shifted to a lower wavenumber than for the the Department of Geosciences, Swedish Museum of Natural related minerals bitikleite and usturite (Galuskina et al., − History, Stockholm, Sweden (20 kV, beam size 1 µm, work- 2010b). Similarly, the strong band at 485 cm 1 related to + ing distance 10 mm). bending vibrations of tetrahedral Fe3 –O is also shifted to lower wavenumbers. Bands at lower wavenumbers (at 269 cm−1 and the nearby right shoulder) may be attributed to translational motion of the tetrahedral Z position and 5 Raman spectroscopy translational motions of Ca–O in the X position. There are no obvious bands that belong to symmetric or asymmetric Laser-induced Raman measurements were carried out at stretching vibrations of Sb5+–O units; according to Guillén- room temperature using a Horiba (Jobin Yvon) LabRAM HR Bonilla et al. (2014), who investigated synthetic byströmite- Evolution hosted at the Department of Earth Sciences, Uni- type CoSb O , there should be manifestation of stretch- versity of Gothenburg, Gothenburg, Sweden. The samples 2 6 ing and coupling at ∼ 518 cm−1, asymmetric stretching at were excited with an air-cooled frequency-doubled 532 nm ∼ 617 cm−1 and symmetric stretching at ∼ 691 cm−1. None Nd:YAG laser utilizing an Olympus 100× objective (numer- of these features could be identified with the exception ical aperture = 0.9). The lateral resolution of the unpolar-
Eur. J. Mineral., 32, 77–87, 2020 www.eur-j-mineral.net/32/77/2020/ A. Karlsson et al.: Monteneveite, a new Sb-bearing member of the garnet supergroup 81
Table 3. Mössbauer hyperfine parameters for monteneveite, at 298 and 77 K.
298 K, χ2 = 1.0 Assignment CS QS FWHM I (%) ZFe3+ I 0.17(2) 1.07(2) 0.31(3) 45(3) ZFe3+ II 0.17(2) 0.62(2) 0.28(2) 33(3) Fe2+/3+ 0.55(2) 0.56(2) 0.27(3) 17(2) ZFe2+ 0.84(3) 2.48(6) 0.23(8) 6(2)
Figure 4. Raman spectrum of monteneveite from 1600 to 100 cm−1 77 K, χ2 = 0.8 with the most prominent peaks indicated. Assignment CS QS FWHM I (%) ZFe3+ I 0.26(1) 1.15(2) 0.22(3) 29(4) of the last one, which may correspond to the shoulder of ZFe3+ II 0.25(1) 0.74(2) 0.29(2) 50(4) + + the 729 cm−1 band in monteneveite. Additionally, Bahfenne Fe2 /3 0.66(2) 0.53(4) 0.30(5) 12(2) + and Frost (2010) reported roméite-group minerals to have a ZFe2 1.01(2) 2.79(3) 0.22(4) 9(2) −1 strong band from 505 to 518 cm related to the symmet- Notes: CS (centre shift), QS (quadrupole splitting), and FWHM (full width at ric stretching mode of octahedrally coordinated Sb, but this half maximum) given in millimetres per second, and I is area as a percentage feature is absent in the Raman spectra of monteneveite, or of the total Fe at that site and valency. possibly quite weak and therefore obscured in the shoulder by the 485 cm−1 band. There are no bands visible in the OH-stretching vibration region (3000–3800 cm−1). There- perfine parameters, in full agreement with numerous Möss- fore, we have concluded that there is no significant hydrog- bauer studies of natural (mostly schorlomite; Amthauer et arnet component (O4H4) that could contribute to charge bal- al., 1977; Schwartz et al., 1980; Wu and Mu, 1986; Lo- + + ance. cock et al., 1995) and Fe3 - and Sb5 -bearing synthetic (Berry et al., 1996) garnet samples. To deal with the line broadness (ca. 0.40 mm s−1 FWHM), a distribution of the 6 Mössbauer spectroscopy quadrupole splitting is assumed, in practice treated by intro- ducing a pair of doublets in the fitting. This approach was About 40 small (diameter ranging from 50 to 300 µm) crys- also used by Berry et al. (1996) for the subspectrum in syn- 5+ 3+ 3+ tals of monteneveite were handpicked from a crushed rock thetic {YCa2}[Sb Fe ](Fe )3O12. chip and mounted on carbon tape in order to validate their Of the two spectra of monteneveite collected, the one at identity with SEM-EDS analysis; they were then turned over 77 K (Fig. 5b) is the most well resolved. Specifically, a rela- and their chemistry checked again to make sure the Möss- tively weak doublet is evident by its absorption on the high- bauer spectra would be essentially free from other phases. velocity side, and it is fitted to be centred at ∼ 1.0 mm s−1 The crystals were ground in an agate mortar and mixed and with a large quadrupole splitting (∼ 2.8 mm s−1). We in- with thermoplastic resin and pressed into a 13 mm tablet. terpret this as originating from the tetrahedrally coordinated The experiments were carried out employing a conventional Fe2+ (blue line in Fig. 5b). The parameters are in good agree- spectrometer system (hosted at the Department of Geo- ment with those found for some natural garnet samples at sciences, Swedish Museum of Natural History, Stockholm, cryogenic temperatures (Locock et al., 1995; Chakhmoura- Sweden) operated in constant-acceleration mode with a 57Co dian and McCammon, 2005) and generally for Fe2+ occu- γ -radiation source (nominally 1.8 GBq). The spectra were pying regular tetrahedra in oxides (e.g. Larsson et al., 1994; collected in 54.7◦ geometry at 298 and 77 K (with a liquid- Holtstam, 1996). In the present spectra, the appearance of N2 cryostat), during 2 weeks each over 1024 channels cov- this peak is clearly temperature-dependant, as it is less pro- ering the velocity range from −4.2 to +4.2 mm s−1. Spec- truding in the 298 K spectrum (Fig. 5a) and therefore more trum calibration was performed against Fe foil, and analy- difficult to constrain. In addition to these two absorption fea- sis was carried out assuming Lorentzian line shapes with the tures, a relatively weak doublet with small quadrupole split- MossA program (Prescher et al., 2012). Relative absorption ting needs to be introduced in the fitting. It has an intermedi- areas and hyperfine parameters obtained for monteneveite in ate centroid shift value (0.66 mm s−1 at 77 K) that cannot be the final fitting procedure are given in Table 3. attributed with confidence to either octahedrally coordinated Both spectra (Fig. 5a, b) are dominated by a doublet with Fe3+ or Fe2+. With respect to garnet structures, data from the a broad absorption centred around 0.17 (298 K) and 0.25 literature are not homogenous, with centroid shifts (at low (77 K) mm s−1. This feature is assigned to the dominant Fe3+ temperatures) usually higher than 1.2 mm s−1 and lower than at the tetrahedrally coordinated Z site, based on the hy- 0.50 for six-coordinated Fe2+ and Fe3+ (Amthauer et al., www.eur-j-mineral.net/32/77/2020/ Eur. J. Mineral., 32, 77–87, 2020 82 A. Karlsson et al.: Monteneveite, a new Sb-bearing member of the garnet supergroup
Table 4. Measured and calculated X-ray powder diffraction data (d in Å) for monteneveite. The strongest diffraction lines are given in bold.
h k l dmeas I/I0meas dcalc I/I0calc 2 2 0 4.45 100 4.4581 100 4 0 0 3.15 60 3.1523 67 4 2 0 2.814 40 2.8195 34 3 3 2 – – 2.6883 4 4 2 2 2.571 80 2.5739 87 4 3 1 – – 2.4729 4 5 2 1 – – 2.3021 6 4 4 0 – – 2.2290 6 6 1 1 – – 2.0455 4 6 2 0 1.993 40 1.9937 27 6 4 0 1.747 15 1.7486 13 6 4 2 1.683 60 1.6850 73 8 0 0 1.575 20 1.5762 13 8 2 2 1.485 10 1.4860 8 8 4 0 1.409 20 1.4098 18 8 4 2 1.375 5 1.3758 5 6 6 4 1.343 20 1.3442 14 8 4 4 – – 1.2869 5 8 6 2 1.235 5 1.2364 5 10 4 2 1.150 15 1.1511 16 8 8 0 1.114 5 1.1145 7 Figure 5. Transmission 57Fe Mössbauer spectrum of monteneveite 8 8 4 1.050 5 1.0508 5 measured at 298 K, χ2 = 1.0 (a) and 77 K, χ2 = 0.8 (b). Both green 12 2 2 – – 1.0227 6 and red lines correspond to [4]Fe3+ while [4]Fe2+ is represented 10 6 4 1.022 15 1.0227 8 by the blue line. The yellow line cannot directly be attributed to 12 6 2 0.930 5 0.9296 8 either octahedrally coordinated Fe3+ or Fe2+ and may represent 8 8 8 0.910 5 0.9100 3 a delocalized electronic state, which could explain why the 77 K Note: calculated diffraction pattern obtained with the atomic spectrum is better resolved. parameters reported in Table 5 (only reflections with I/I0calc ≥ 3 are listed).
1977; Wu and Mu, 1986; Locock et al., 1995; Chakmoura- actly calculate the amount of Fe2+, but the fraction obtained dian and McCammon, 2005). In fact, the value here is more at 77 K (9 %) is not too far from the expected value (12 %). characteristic for a delocalized electronic state, correspond- This approach also assumes similar recoil-free fractions for ing to a charge intermediate between the formal 2+ and Fe2+ and Fe3+ at the different sites, which may be only ap- 3+. A charge-transfer process involving [4]Fe2+ and [6]Fe3+ proximately true. could explain this feature and agrees well with the observed temperature behaviour; thermally induced electron hopping [ ] + decreases, and hence we see the 4 Fe2 doublet develop to- 7 X-ray crystallography wards lower temperatures at the expense of the absorption related to the delocalized state of Fe valence electrons (Ta- 7.1 Powder X-ray diffraction data ble 3). There is no general consensus that intervalence charge X-ray powder diffraction data (Table 4) were obtained on transfer occurs in garnet structures for Fe2+ and Fe3+ hosted the fragment used for the single-crystal study (see below) in corner-sharing tetrahedra and octahedra, although it is has with an Oxford Diffraction Xcalibur PX Ultra diffractome- been inferred from Mössbauer data in some studies (Hug- ter fitted with a 165 mm diagonal Onyx CCD detector and gins et al., 1977; Wu and Mu, 1986; Chakmouradian and using copper radiation (CuKα, λ = 1.54138 Å). The work- McCammon, 2005). It should be noted that the amount of ing conditions were 40 kV and 40 nA with 1 h of expo- tetrahedrally coordinated Fe3+, calculated to 82 % of total sure; the detector-to-sample distance was 7 cm. The program Fe from the EPMA-based formula, shows a good agree- CrysAlis RED was used to convert the observed diffraction ment with band area ratios from the Mössbauer data, 79(4) rings to a conventional powder diffraction pattern. Least- and 78(3) % at 77 and 298 K, respectively. Considering the squares refinement gave the following unit-cell values: a = 3 charge-transfer phenomena, it would not be possible to ex- 12.6032(2) Å and V = 2001.9(1) Å .
Eur. J. Mineral., 32, 77–87, 2020 www.eur-j-mineral.net/32/77/2020/ A. Karlsson et al.: Monteneveite, a new Sb-bearing member of the garnet supergroup 83
Table 5. Data and experimental details for the selected monten- were merged according to the m3m Laue group (Rint = eveite crystal. 2.71 %). The crystal structure was refined in the Ia3d start- ing from the atomic coordinates of andradite (Hazen and Fin- Crystal data ger, 1989). Scattering curves for neutral atoms were taken 3+ 2+ from the International Tables for Crystallography (Wilson, Ideal formula Ca3Sb2Fe Fe O12 2 1992). During the first refinement cycles, the occupancy at 3 Crystal size (mm ) 0.065 × 0.110 × 0.180 the X and Z sites was allowed to vary (Ca vs. and Fe Form Block vs. ). The refined mean electron numbers were 20.0(2) and Colour Black − Crystal system Cubic 26.0(2) e for the X and Z sites, respectively, and therefore Space group Ia3d their site occupancy factors (sof’s) were fixed in the subse- a (Å) 12.6093(2) quent stages of the refinement. However, the sof at the Y V (Å3) 2004.8(1) site was allowed to vary (Sb vs. Fe). After several cycles Z 8 of anisotropic refinement, a final R1 = 0.0197 for 305 re- Data collection flections with Fo > 4σ (Fo) with 19 refined parameters was achieved (0.0286 for all 401 reflections). Atomic coordi- Instrument Oxford Diffraction Xcalibur 3 Radiation type Mo Kα (λ = 0.71073 Å) nates, site occupancies and equivalent isotropic displacement Temperature (K) 293(3) parameters are given in Table 6. Selected bond distances and Detector to sample distance (cm) 5 bond-valence sums (calculated according to the parameters Number of frames 685 of Brese and O’Keeffe, 1991) are given in Table 7. Measuring time (s) 10 Maximum covered 2θ (◦) 73.06 Absorption correction multi-scan (Oxford Diffraction, 2006) 8 Results and discussion Collected reflections 7947 Unique reflections 401 8.1 Crystal chemistry Reflections with Fo > 4σ (Fo) 305 Rint 0.0271 The crystal structure of monteneveite (Fig. 6) is identi- Rσ 0.0116 Range of h, k, l −20 ≤ h ≤ 18, −18 ≤ k ≤ 20, cal to that of the cubic members of the garnet supergroup −17 ≤ l ≤ 20 (Grew et al., 2013). The overall chemical formula, as ob- tained through the single-crystal X-ray diffraction study is Refinement Ca3[Sb0.93Fe0.07]2Fe3O12 (Z = 8), in excellent agreement Refinement Full-matrix least squares on F 2 with that obtained from EPMA data (overall mean elec- Final R1 (Fo > 4σ (Fo)) 0.0197 tron number from X-ray data = 236.5; overall mean electron Final R1 (all data) 0.0286 number from chemical data = 236.4). Such a cation distribu- S 1.137 Number refined parameters 19 tion is in agreement with the observed bond distances. The 4+ −3 1ρmax (e Å ) 0.58 minor amounts of Sn at the octahedrally coordinated site −3 1ρmin (e Å ) −0.78 (Y) and Zn at the tetrahedrally coordinated site (Z) were as- signed on the basis of the preference of these cations shown in toturite or irinarassite (Galuskina et al., 2010a, 2013b) and yafsoanite (Mills et al., 2010), respectively. The tetrahedron 7.2 Single crystal X-ray diffraction data shows a mean Z–O distance of 1.888(1) Å, slightly longer than that expected for pure Fe3+–O, which, on the basis of A single fragment of monteneveite EXAFS measurements, was estimated to be close to 1.85 Å (0.065 mm × 0.110 mm× 0.180 mm) was examined with (Giuli et al., 2012); on the other hand, < Z-O> in monten- an Oxford Diffraction Xcalibur single-crystal diffrac- eveite is shorter than the corresponding value (1.905 Å) ob- 3+ tometer equipped with a CCD detector, with graphite- served in the synthetic Ca3Sb2Fe2 ZnO12 garnet (Bhim et monochromatized MoKα radiation (λ = 0.71073 Å). The al., 2017), in keeping with a content of tetrahedral divalent collected data were integrated and corrected for standard cations less than 1.00 atoms pfu. The presence of measur- Lorentz and polarization factors with the CrysAlis RED able quantities of divalent Fe at the Z site in a garnet struc- package (Oxford Diffraction, 2006). The software AB- ture is not too common, but it has been demonstrated with SPACK in CrysAlis RED (Oxford Diffraction, 2006) was spectroscopic methods, both for synthetic material (e.g. An- used for the absorption correction. Table 5 reports details of tonini et al., 1981) as well as in natural samples (Amthauer the selected crystal, intensity data collection and refinement. et al., 1977; Locock et al., 1995). Deviations from the cubic symmetry of the unit-cell val- The dominance of Sb5+ at the six-coordinated Y site is in ues were well below their standard deviation. A cubic unit good agreement with the mean bond distance [1.990(1) Å], cell [a = 12.6093(2) Å] was assumed and the intensity data slightly larger than the pure octahedral
Table 6. Atoms, site occupancy factors (sof), fractional atomic coordinates and equivalent isotropic displacement parameters (Å2) for the selected monteneveite crystal.
Atom sof x y z Ueq
X Ca1 1/8 0 1/4 0.0082(2) Y Sb0.932(3) 0 0 0 0.0047(1) Fe0.068 Z Fe1 3/8 0 1/4 0.0066(2) OO1 0.0288(1) 0.0501(1) 0.6469(1) 0.0113(3)
Table 7. Bond distances (Å) and bond valence sums (BVS in v.u.) (2.425 Å, X = Ca, Z = Si3; Hazen and Finger, 1989), or even = = 3+ 2+ in the crystal structure of monteneveite. in schorlomite (2.440 Å, X Ca, Z Si2.35Fe0.34Fe0.31; Pe- terson et al., 1995). For the same reason, its degree of distor- X–O (×4) 2.419(1) tion is very low [1(X–O) = 0.144 Å]. X–O (×4) 2.562(1) It can be noted that the microhardness of monteneveite is BVS 1.98 only slightly lower than that of andradite (11.2 vs. 11.5 GPa), Y –O (×6) 1.990(1) which is in agreement with the general picture that the larger BVS 5.07 the volume of the XO8 polyhedron, the higher the com- × Z–O ( 4) 1.888(1) pressibility of the garnet structure (Milman et al., 2001). As BVS 2.37 an example, grossular, which shows an
8.2 Remarks on nomenclature
5+ 3+ 2+ Monteneveite, ideally {Ca3}[Sb2 ](Fe2 Fe )O12, belongs to the garnet supergroup (Grew et al., 2013) and is so far the only known member of the garnet supergroup with nom- 3+ 2+ inal mixed (R2 R ) population at the tetrahedral Z site. The other garnet minerals having a total charge at Z site = 8 Figure 6. The monteneveite crystal structure seen along [100]. are henritermierite (Armbruster et al. 2001) and holtstamite Blue spheres– X site (Ca); green octahedra– Y site (Sb); brown (Hålenius et al., 2005), which, however, achieve the total tetrahedra– Z site (Fe). 4+ charge by a mixed (R2 ) population. On the mere basis of total charge at the Z site, monteneveite should be classi- from literature: 1.9647 Å in roméite (Matsubara et al., 1996), fied into the henritermierite group. Nonetheless, the crystal- 1.960–1.983 Å in ingersonite (Bonazzi and Bindi, 2007), lochemical properties strongly differ from those of the hen- 1.958 Å in filipstadite (Bonazzi et al., 2013) and 1.971– ritermierite group minerals on some critical points: 2.022 Å in rinmanite (Holtstam et al., 2001). Due to the very large volume of the tetrahedron, the X dodecahedron (which 1. monteneveite is cubic, whereas henritermierite and holt- shares two edges with it) shows a mean distance (2.490 Å) stamite crystallize in the tetragonal system, space group much larger than that observed, for example, in andradite I41/acd;
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Table 8. Comparison of monteneveite to other related species in the garnet supergroup.
Name End-member formula Charge at Z Space group a (Å) c (Å) 1 5+ 3+ 2+ Monteneveite {Ca3}[Sb2 ](Fe2 Fe )O12 8 Ia3d 12.6093(2) 2 5+ 4+ Bitikleite {Ca3}[Sb Sn ](Al3)O12 9 Ia3d 12.5240(2) 3 5+ 3+ Usturite {Ca3}[Sb Zr](Fe3 )O12 9 Ia3d 12.49 4 5+ 4+ 3+ Dzhuluite {Ca3}[Sb Sn ](Fe3 )O12 9 Ia3d 12.536(3) 5 6+ 3+ Elbrusite {Ca3}[U0.5Zr1.5](Fe3 )O12 9 Ia3d 12.7456(9) 6 3+ Henritermierite {Ca3}[Mn2 ](Si2)()O8 (OH)4 8 I41/acd 12.489(1) 11.909(1) 7 Holtstamite {Ca3}[Al2] (Si2)()O8 (OH)4 8 I41/acd 12.337(3) 11.930(5)
1 This work. 2 Galuskina et al. (2010b). 3 Galuskina et al. (2010b). 4 Galuskina et al. (2013a). 5 Galuskina et al. (2010c). 6 Armbruster et al. (2001). 7 Hålenius et al. (2005).
5+ 3+ 2+ ± 2. the general formula is Ca3R2 (R2 R )O12 instead of ture; the amount is even less than the 0.07 0.04 wt % SiO2 3+ 4+ found for a specimen of manganberzeliite (Nagashima and Ca3R2 (R2 )O12; Armbruster, 2012). 3. monteneveite is not a silicate.
Furthermore, in spite of the chemical similarities (see Ta- Data availability. All data needed to draw the conclusions in the ble 8) and identical symmetry (space group Ia3d), mon- present study are shown in this paper. The CIF is available in the teneveite does not fit with the bitikleite group (charge at Z Supplement. site = 9). For these reasons and because the subdivision of a group into subgroups is strongly discouraged (Grew et al., 2013), it seems appropriate to introduce a new group – pro- Supplement. The supplement related to this article is available on- vided that other distinct members of this type are discovered line at: https://doi.org/10.5194/ejm-32-77-2020-supplement. 5+ 3+ (e.g. {Ca3}[Sb2 ](Fe2 Zn)O12). In a general mineral classi- fication (Nickel–Strunz), monteneveite belongs to the oxide class, subdivision 4.CC. Author contributions. AK found the mineral and performed the ini- tial chemical analyses. AK and MKS collected micro-Raman data, 8.3 Origin of monteneveite and AK and DH interpreted the spectrum. LB and PB did the single- crystal X-ray diffraction experiment and refined the crystal struc- Monteneveite is most likely a product of skarn formation ture. AK and DH performed Mössbauer spectroscopy and inter- during regional metamorphism. The partial breakdown of preted the data. MKS collected electron-microprobe data and AK tetrahedrite-(Fe) and recrystallization of calcite would gen- interpreted the results. AK and DH wrote the paper with contribu- erate the chemical constituents needed to form both mon- tions from all coauthors. teneveite and oxycalcioroméite. Large parts of the sulfide as- semblages were probably formed during early diagenetic and Competing interests. The authors declare that they have no conflict low-metamorphic conditions. At peak metamorphism these of interest. were resorbed and partially recrystallized. The source of Sn is unknown, but the concentrations observed in monteneveite and oxycalcioroméite might originally have been mobilized Acknowledgements. Special thanks to Christina Günter, University from the partial dissolution of stannite, not observed in the of Potsdam, for her help with the EPMA work and to Torbjörn Lorin present sample but reported from the deposit by Mair et for the colour photograph of the specimen. Additional thanks go to al. (2007). Mineralogical evidence suggests that f O2 was Henrik Skogby for help with preparing the sample for Mössbauer within the magnetite stability field since magnetite is present spectroscopy. Luca Bindi and Paola Bonazzi thank CRIST, Labo- both as idiomorphic crystals contiguous to monteneveite and ratorio di Cristallografia Strutturale, University of Florence, Italy. as inclusions. It is probable that upper-amphibolite-facies This paper benefited from reviews by E. Galuskin and the anony- metamorphic conditions (∼ 600 ◦C and 8–10 kbar) would be mous journal reviewer. required in order to form monteneveite, which is further favoured by a Si- and Al-poor local environment. To the best of our knowledge, monteneveite is the first completely Si- Review statement. This paper was edited by Sergey Krivovichev. free natural garnet species (not considering the fluoride cry- olithionite) at the detection level of EPMA used in our study (∼ 0.035 wt %) compared to what is reported in the litera- www.eur-j-mineral.net/32/77/2020/ Eur. J. Mineral., 32, 77–87, 2020 86 A. Karlsson et al.: Monteneveite, a new Sb-bearing member of the garnet supergroup
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